With the great advance of nanotechnology, there is an increasing demand for rapid parallel manufacturing strategies for nanostructures like nano-holes and nano-pillars. Some applications that require repetitive (periodic) uniform nano-holes and nano-pillars over large areas are photonic crystals, memory devices, OLED nano-filtration, solar cells, artificial kidney, etc.
Conventional photolithography techniques cannot satisfy the requirements of the nano-patterns, due to the wavelength limit of the light source used. Novel techniques like X-ray, electron beam, and focused ion beam techniques are either slow or expensive for manufacturing such repetitive (periodic) patterns over large areas. Highly monodisperse micro- and nano-spheres can self-organize to form a hexagonally close packed (HCP) self-assembled monolayer and have attracted widespread attention for making large area periodic nanostructures. One important example is the Nanosphere Lithography (NSL) technique, which uses planar ordered arrays of micro/nanospheres as a lithography mask to generate ordered nanoscale arrays on a substrate.
Document “Fabrication of Large Area Periodic Nanostructures Using Nanosphere photolithography”, Wei Wu et al, Nanosclae Res. Lett. (2008) 3, pp. 351-354 disclosesfor instance a method that uses a photoresist layer covered with a self-assembled ordered monolayer of hexagonally close packed silica micro-spheres (each micro-sphere having a diameter of 1 micrometer). The photoresist layer is exposed UV radiations (centered at a wavelength of about 400 nanometers) through the monolayer of micro-spheres. During exposure, each micro-sphere acts as a micro-lens, focusing the light radiations on the photoresist layer so as to generate patterns of nano-pillars having a diameter of about 180 nanometers and a periodicity of 1 micrometer in the phororesist layer. Gold and aluminum films having nano-holes are formed by lift-off on the photoresist nanopillars.
However, in such a mask-less method, the micro-spheres must be deposited on the photoresist layer before exposure to UV radiations, and then removed afterward, in a wet process. Therefore, the micro-spheres cannot be reused and deposition of the self-organized microspheres must be repeated for each sample preparation.
Document “Fabrication of arrays of two-dimensional micropatterns using microspheres as lenses for projection photolithography”, Ming-Hsien Wu et al, Applied Phys Letters, vol. 78, no. 16, pp. 2273-2275, discloses a method for forming a repetitive, micrometer-scale pattern in a photoresist layer using an array of polystyrene microspheres (having a diameter of 1.5 micrometers) embedded in a poly(dimethylsiloxane) membrane. The transparent microspheres act as lenses which project the image of an illuminated mask on the photoresist layer. The thickness of the membrane is chosen so that, when the membrane is held in conformal contact with the photoresist layer, the microspheres are held at an appropriate distance from the photoresist, corresponding to the focal length of a microsphere. After exposure to light radiations, the membrane is peeled from the photoresist layer and the photoresist layer is developed in a solution of sodium hydroxide.
Such a method allows generating a repetitive, micrometer-scale pattern in the photoresist, the pattern having a spatial period which is equal to the diameter of the microspheres, i.e. about 1 or 1.5 micrometers. As the spatial period of the resulting pattern is equal to the diameter of the microspheres, adjustment of the spatial period may only be obtained by using micro-spheres of different sizes.